1. Field of the Invention
The present invention relates generally to a method and apparatus for estimating a Doppler frequency in a mobile terminal, and more particularly to a method and apparatus for estimating a Doppler frequency independent of a channel estimation error in a mobile terminal.
2. Description of the Related Art
In order to provide high-speed multimedia packet services through limited channel resources in a mobile communication system, it is important to allocate system resources using accurate channel information. The speed of a mobile terminal is a very important factor of the channel information. For example, the speed of a mobile terminal is used to determine a coefficient of a receiver, such as a channel estimation length or an interleaver size, by most adaptive algorithms used in mobile communication environments. The speed of a mobile terminal is also used for power control or handover of the mobile terminal.
While moving, the mobile terminal experiences a so-called Doppler shift, i.e., a frequency shift of a received (RX) signal. The Doppler shift causes a frequency offset of an RX signal in proportion to the speed of the mobile terminal relative to a Base Station (BS). Thus, based on the fact that the speed of a mobile terminal is proportional to a frequency error of an RX signal caused by a Doppler shift, conventional technologies estimate a maximum Doppler frequency of an RX signal to estimate the speed of a mobile terminal. For example, conventional methods estimate the maximum Doppler frequency using Level Crossing Rate (LCR) and Zero Crossing Rate (ZCR) characteristics, an autocorrelation function value of an RX signal, a Fast Fourier Transform (FFT), or a covariance of the square of an RX signal strength.
If it is assumed that a complete channel is estimated in a noise-free environment by jakes modeling of a mobile communication channel, an autocorrelation function of a wireless channel h(n) can be expressed as shown Equation (1):Rhh(k)=E[h(n)h*(n+k)]=σh2J0(2πFDTsk),  (1)where E[x] denotes an expected value of x, Ts denotes a sampling period, fD denotes a maximum Doppler frequency, σh2 denotes the power of a channel h(n), and J0( ) denotes a zero-order Bessel function of the first kind.
Rhh(0) and Rhh(1) are determined from Equation (1) and then the maximum Doppler frequency fD is determined as shown in Equation (2):
                                          f            D                    =                                    1                              π                ⁢                                                                  ⁢                                  T                  s                                                      ⁢                                          1                -                                                                            R                      hh                                        ⁡                                          (                      1                      )                                                                                                  R                      hh                                        ⁡                                          (                      0                      )                                                                                                          ,                            (        2        )            where Ts denotes a sampling period and Rhh(0) and Rhh(1) denote autocorrelation functions of wireless channels.
Equation (2) is the estimation of the maximum Doppler frequency on the assumption that a complete channel was estimated in a noise-free environment. However, noises that are actually present in a channel of a real wireless environment make it difficult to accurately estimate the channel, thus degrading system performance.
When channel estimation is inaccurate, due to a noise, for example, an estimated channel can be expressed as shown in Equation (3):h(n)=ρ{circumflex over (h)}(n)+e(n),  (3)where h(n) denotes an estimated channel, ρ denotes a normalized correlation coefficient between the original channel and the estimated channel, and e(n) denotes a white noise with a mean of 0 and a variance of 1−|ρ|2.
When the estimated channel is expressed as Equation (3), an autocorrelation function can be expressed as shown in Equation (4). Herein, like in the error-free case, the maximum Doppler frequency is expressed by R(0) and R(1) as shown in Equation (5) below.R(k)=σh2|ρ|2J0(2πfDTsk)+(1−|ρ|2)δ(k)  (4)
In Equation (4), R(k) denotes an autocorrelation function for k samples, Ts denotes a sampling period, fD denotes a maximum Doppler frequency, σh2 denotes the power of a channel h(n), J0( ) denotes a zero-order Bessel function of the first kind, and δ(k) denotes a delta function.
                              f          D                =                              1                          π              ⁢                                                          ⁢                              T                s                                              ⁢                                    1              -                                                1                                                                                  ρ                                                              2                                                  ⁢                                  (                                      1                    +                                                                  1                        -                                                                                                          ρ                                                                                2                                                                                            σ                        h                        2                                                                              )                                ⁢                                                      R                    ⁡                                          (                      1                      )                                                                            R                    ⁡                                          (                      0                      )                                                                                                                              (        5        )            
In Equation (5), Ts denotes a sampling period, fD denotes a maximum Doppler frequency, σh2 denotes the power of a channel h(n), and R(k) denotes an autocorrelation function for k samples.
A conventional method of estimating a maximum Doppler frequency using an autocorrelation function as described above is effective only when aware of accurate noise power. When the accurate noise power is unavailable, the conventional method cannot accurately estimate the speed of a mobile terminal, thus causing performance degradation. That is, as illustrated in FIG. 1, as a Signal-to-Noise Ratio (SNR) decreases with an increase in noise error, an estimation error in the maximum Doppler frequency increases, thus increasing an estimation error in the speed of the mobile terminal.